1. Field of the Invention
The present invention relates to the field of solar cell semiconductor devices, and particularly to integrated semiconductor structures including a multifunction solar cell and a bypass diode.
2. Description of the Related Art
Photovoltaic cells, also called solar cells, are one of the most important new devices for producing electrical energy that has become commercially competitive with other energy sources over the past several years. Considerable effort has gone into increasing the solar conversion efficiency of solar cells. As a result, solar cells are currently being used in a number of commercial and consumer-oriented applications. While significant progress has been made in this area, the requirement for solar cells to meet the needs of more sophisticated applications has not kept pace with demand. Applications such as satellites used in data communications have dramatically increased the demand for solar cells with improved power and energy conversion characteristics.
In satellite and other space related applications, the size, mass and cost of a satellite power system are dependent on the power and energy conversion efficiency of the solar cells used. Putting it another way, the size of the payload and the availability of on-board services are proportional to the amount of power provided. Thus, as the payloads become more sophisticated, solar cells, which act as the power conversion devices for the on-board power systems, become increasingly more important.
Solar cells are often fabricated from semiconductor wafers in vertical, multifunction structures, and the wafers or cells are laid out in a planar array, with the individual solar cells connected together in columns in a series electrical current. The shape and structure of the columns forming the array, as well as the number of cells it contains, are determined in part by the desired output voltage and current.
When solar cells in an array are all receiving sunlight, i.e. the top layer of the cell is illuminated, each cell in the array will be forward biased and will be carrying current. However, when the solar cell is not receiving sunlight, whether because of shading by a movement of the satellite or as a result of damage to the cell, then resistance exists along the cell path. As solar cells exist in an array, some cells may be generating current, and others may be inactive. In such case, the current from illuminated cells must still pass through the shaded cells. A current would force its way through the cell layers, reversing the bias of such cells and permanently degrading, if not destroying the electrical characteristics of such cells.
If the series electrical circuit contains a diode and certain solar cells are shaded, the current passing through the shaded cells can be offered an alternative, parallel path through the inactive cells, and the integrity of the shaded cells will be preserved. The purpose of the bypass diode is to draw the current away from the shadowed or damaged cell. The bypass diode become forward biased when the shadowed cell becomes reverse biased. Since the solar cell and the bypass diode are in parallel, rather than forcing current through the shadowed cell, the diode draws the current away from the shadowed cell and completes the electrical current to maintain the connection to the next cell.
If a cell is shaded or otherwise not receiving sunlight, in order for the current to choose the diode path, the turn on voltage for the diode path must be less than the breakdown voltage along the cell path. The breakdown voltage along the cell path will typically be at least five volts, if not more. In the case of a Schottky bypass diode, the Schottky contact requires a relatively small amount of voltage to “turn on”, about 600 millivolts. However, to pass through the Ge junction the bias of the Ge junction must be reversed, requiring a large voltage. Reversing the bias of the Ge junction requires approximately 9.4 volts, so nearly ten volts are needed for the current to follow the diode path. Ten volts used to reverse the bias of the Ge junction is ten volts less than otherwise would be available for other applications.
U.S. Pat. No. 6,680,432 describes a multijunction solar cell with an integral bypass diode structure in which a metal shunt is used to “short” the Ge junction to the base of the bypass diode. Because of the short, a minimal voltage is required to pass current between the bypass diode and the Ge substrate. No longer is a high voltage required to force the current through the Ge junction. The current flows easily through the “short” path.
More particularly, the multijunction solar cell described in the above noted patent includes a substrate, a bottom cell, a middle cell, a top cell, a bypass diode, a lateral conduction layer, and a shunt. The lateral conduction layer is deposited over the top cell. The bypass diode layers are deposited over the lateral conduction layer. In one portion of the substrate, the bypass diode layers are removed, leaving the exposed solar cell layers. In the other portion, the layers to be used in forming the bypass diode are allowed to remain. A trough is etched, electrically separating the solar cell region from the by pass region. A metal shunt layer is deposited with one side of the shunt connected to the substrate and another side of the shunt connected to the lateral conduction layer which connects to an active layer of the bypass diode. The metal shunt acts to short the intermediate layers forming the support of the bypass diode, so that such layers do not perform any electrical function, but only act as the support of the bypass diode.
As noted above, individual solar cells are connected sequentially to form a vertical column of an array. Such series connection requires an electrical path between the cathode or top layer of one cell with the anode or bottom layer of the adjacent cell. In particular, in solar cells with an integral bypass diode, a connection must be made from both the multijunction solar cell and from the bypass diode on the top surface of a first wafer to the bottom surface of the adjoining wafer.
Prior art interconnection arrangements have utilized a single electrical contact to the top layer (or anode) of the bypass diode. Although such an arrangement is generally satisfactory for most applications and reliability requirements, there are certain applications in which more stringent reliability is required. Prior to the present invention, existing interconnection arrangements have not been able to meet such reliability requirements.